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This report is not referring to the background in a scene at all. The unrendered region should simply always be transparent — after all, as you correctly pointed out, it is not rendered. It should not be filled with black.

I think it is reasonable to expect unrendered portions of the image to come out transparent when alpha support is enabled for the output file. After all, the preview window also still shows the checkerboard pattern in those regions. I therefore re-opened this issue as a feature request.

I think it is reasonable to expect unrendered portions of the image to come out transparent when alpha support is enabled for the output file. After all, the preview window also still shows the checkerboard pattern in those regions. I therefore re-opened this issue as a feature request.

I agree, it would be a useful feature. Of course, we had been discussing if we want a full image generated for a partial render or not for a long time anyway. Once we add a choice for that, we can also change the current behavior.

Just an opinion to consider: outputting a partial render with transparent unrendered parts can be extremely useful. In cases when a small part of the image changes, and the rest stays the same (this is ok if there's no radiosity and reflections), rendering speed can be significantly increased if only the changing part is rendered, and then the partial image is composited over a single render of surroundings. Let's say, a render of animation that includes a rotating or morphing object in static environment (common in scientific presentations). Having a transparent background makes the compositing a trivial operation. The same technique can be used to distribute rendering across different machines (such as ad-hoc cluster over ssh): chunks can be easily merged if the unrendered parts are transparent.